1. Field of the Invention
The present invention relates to semiconductor manufacturing techniques, and more specifically, relates to a semiconductor device and a manufacturing method thereof.
2. Description of the Related Art
With the improvement of semiconductor manufacturing techniques, critical dimensions of a semiconductor device continue to scale down. In such cases, characteristics (e.g., contact resistance) of a contact has a great impact on device performance (such as, drive current Ion degradation).
When metals are used for forming a contact plug between metal wiring layers, it is necessary to select metal materials according to actual requirements, since each metal may have its own advantages and disadvantages. However, if two kinds of metals are properly used in combination, their respective disadvantages may be avoided while the advantages are further enhanced, so that better performance can be obtained.
The present invention uses commonly-used metals (i.e., tungsten and copper) as examples for the following description.
When conventional tungsten (W) is used to form a contact plug, an disadvantage of tungsten is its relatively high resistivity. Much work has been done to improve tungsten plug technology in order to reduce contact resistance (Rc). An example is to decrease the thickness of a barrier metal on the sidewall of a contact hole. Also, the resistivity of the filled tungsten can be reduced by controlling the characteristics of a nucleation layer deposited using WF6 and B2H6 by an atomic layer deposition (ALD) technology. However, the resistivity of tungsten is still much higher than the resistivity of other metals like copper, silver or aluminum.
When copper (Cu) is used to form a contact plug, some issues may arise. One is Cu diffusion that may occur in silicon and oxide, and once Cu diffuses into regions such as a channel region, it will have an adverse impact on device performance. The other is voids formed in a copper contact plug when a Back-End-Of-Line (BEOL) Cu process is used to fill a contact hole of high aspect ratio will increase the contact resistance of the contact plug.
However, if a relatively short lower portion of the contact hole is filled with tungsten while an upper portion of the contact hole is filled with copper, the resistance of the entire contact plug can be reduced, and at the same time, filling a contact hole of high aspect ratio entirely with copper can be avoided, thus preventing Cu from diffusing into a channel region.
Likewise, as for other metals, if one metal is suitable for filling an upper portion of a contact hole but not its lower portion while another metal is suitable for filling the lower portion but not the upper portion according to design requirements of an integrated circuit device, it will be beneficial to fill the upper portion and the lower portion of the contact hole with two metals.
Taking tungsten and copper as examples, a conventional method for forming a contact plugs by combining two different kinds of metals reads as follows.
As shown in FIG. 1, short W contacts 120 are formed in openings of a first dielectric layer (for example, an interlayer dielectric layer (ILD)) 110 on a substrate 100.
Then, as shown in FIG. 2, a second dielectric layer (for example, an ultra-low-k layer (ULK)) 130 is deposited. A dual damascene process is performed to form an upper Cu contact 140 and first layer of metal connecting lines 150.
However, the conventional method requires contact-hole photolithography to be performed twice that has issues of position alignment and high cost.
Therefore, a more convenient and simpler method for forming a contact plug by employing two kinds of metals to respectively fill an upper portion and a lower portion of a contact hole is desirable.